This invention relates generally to the direct reduction of iron oxide materials to produce metallized iron in solid state such as hot metallized pellets or hot sponge iron in a direct reduction shaft furnace. "Metallized" as used throughout this specification and the appended claims means substantially reduced to the metallic state i.e. always in excess of 75% metal, and usually in excess of 85% metal in the product. Such metallized pellets or sponge iron are well suited as feed materials to steel making furnaces such as an electric arc furnace.
Clark et al, U.S. Pat. No. 4,054,444 teaches means for controlling the carbon content of direct reduced iron pellets when discharged cold from a direct reduction shaft furnace. The gas injected in the Clark et al patent is methane, natural gas, or heavy hydrocarbon gas, to which optionally can be added clean spent top gas from the direct reduction furnace. The gas is injected into the buffer zone, which is the zone between the reduction zone and cooling zone in the furnace. One of the functions of the Clark et al invention is to precool the burden before it reaches the cooling zone to reduce the required cooling within the cooling z one. The present invention requires the avoidance of this cooling effect.
Currently, there are three known methods for increasing the carbon content of direct reduced iron product, all of which are implemented in commercial operation. These three methods are:
(1) lowering the reducing gas temperature at the furnace bustle;
(2) increasing the methane or other hydrocarbon content of the reducing gas to the bustle by adding natural gas; and
(3) injecting natural gas into the lower, or discharge section, of the furnace.
Each of these methods increases the carbon content of the direct reduced iron product, but each method also has limitations in normal furnace operation.
Lowering the reducing (bustle) gas temperature has proven to increase the carbon content in the product in operating direct reduction plants around the world, however, the plant production (output) also suffers a reduction, due to slower reducing reactions. This loss of production capacity with lower reducing gas temperatures has been verified by plant operating history over many years.
Increasing the hydrocarbon content of the reducing gas by adding natural gas to enrich the reducing gas at the bustle has been attempted in order to raise the carbon content of the product. The added hydrocarbon in the reducing gas cracks at high furnace temperatures, adding more carbon to the product.
The cracking of these hydrocarbons produces carbon which is integrated into the product, and hydrogen which flows upwardly through the shaft furnace where it acts as additional reductant gas for reducing the iron oxide to metallized iron (or direct reduced iron) in the upper reduction zone of the shaft furnace. The amount of hydrocarbon that can be added to the furnace is limited because the cracking of hydrocarbons is an endothermic reaction. An overabundance of hydrocarbons in the reducing gas, when cracked to form carbon (C) plus hydrogen gas (H.sub.2), causes a cooling trend in the shaft furnace. The resulting reduction in burden temperature causes a slower reduction reaction between the reducing gas and the iron oxide, and, ultimately, lower production. In addition, in a hot discharge/hot briquetting (HD/HB) direct reduction plant, the added cooling adversely affects the ability of the metallized iron product to be briquetted, a situation which must always be avoided.
Injection of natural gas into the lower cone (cooling and discharge) region of the shaft furnace is also a proven method of adding carbon to the product in direct reduction plants. In a cold product discharge plant, this is an excellent and economic method of adding carbon to the product. It is limited only by the amount of cooling that can be tolerated in the upper (reducing) section of the shaft furnace without significantly reducing the furnace output or product quality. The usual desired level of carbon addition to the product can be easily achieved without reaching the point of over-cooling the burden, since it is desirable to discharge the product at near ambient temperatures. In HD/HB plants, an added product specification must be met in addition to production rate and product quality; the product must be sufficiently hot on discharge to be compacted into briquets. It is this product requirement that severely limits the amount of natural gas that can be injected into the lower portion of the hot discharge furnace. The endothermic reaction of cracking the natural gas can cool the burden below the minimum temperature for good briquetting. The three methods described above all have the same limitation of temperature. The reduction temperature in the furnace must be maintained above at least 760.degree. C. if production is to be maintained. In the case of an HD/HB furnace operation, a high discharge temperature (above about 700.degree. C.) must also be maintained to insure good briquetting. This final temperature requirement for hot discharge plants severely hinders the effectiveness of these three methods to deposit the desired amount of carbon in the product.
The problem is twofold: first, to add carbon to the product, and second, to avoid contributing any significant endothermic load to the furnace burden. The present invention overcomes both of these problems by making a controlled addition of a mixture of hot "endothermic gas", enriched with natural gas at ambient temperature, to the furnace discharge zone. Endothermic gas is a hot-air reformed hydrocarbon, produced in a catalytic reformer by reacting a mixture of natural gas and air and/or oxygen. Throughout this specification, the term "endothermic gas" embraces a gas containing a carbon monoxide percentage of from about 20 to about 60%, hydrogen, residual carbon dioxide and water vapor of less than one percent, and nitrogen if the reforming is accomplished by the use of air. Commonly available endothermic gas contains about 20% CO, 40% H.sub.2, less than 0.4% residual H.sub.2 O vapor and CO.sub.2, and the balance of about 40% N.sub.2.
The accomplishment of both of these objectives rests in the fact that the endothermic gas/natural gas mixture forms a "balanced" system, from a heat of reaction standpoint. The disadvantage to adding only natural gas to the furnace is the endothermic cracking reactions that cause cooling within the furnace. In the endothermic gas/natural gas mixture, there is a balancing reaction to the cracking reactions: EQU 2CO (g)=C (s)+CO.sub.2 (g)
This is the Boudouard reaction. This reaction is possible because of the high CO content in the endothermic gas. As the temperature begins to fall in the furnace because of the cooling effect from the cracking of the natural gas, the equilibrium of the Boudouard reaction favors carbon deposition to a greater extent. The deposition of carbon from the Boudouard reaction is an exothermic reaction. Therefore, by mixing the endothermic gas and natural gas in the proper ratio, a balancing of the endothermic and exothermic heat loads in the furnace is realized. As the natural gas cools the burden by cracking, the CO restores the lost heat by decomposing to CO.sub.2 and solid carbon.
The natural gas--endothermic gas mixture is injected into the lower cone of the furnace at temperatures at or above the required minimum temperature to insure good briquetting. This inlet temperature is controlled by the amount of cold natural gas used to enrich the hot endothermic gas. Since the endothermic gas/natural gas mixture to the lower cone is hot, it provides an additional benefit during plant start-up.
The endothermic gas/natural gas mixture provides more carbon than the enriched bustle gas method because of the lower temperature in the lower cone region of the furnace. Bustle gas temperatures are sufficiently high to crack the heavy hydrocarbons in the natural gas, but the temperature is too high for the Boudouard reaction to be carbon depositing. In the lower cone region, the temperatures are lower than bustle gas temperatures. They are cool enough that the Boudouard reaction favors carbon deposition, while still being warm enough to crack the hydrocarbons in the natural gas portion of the mixture. It is this slightly cooler environment in the lower cone region that makes this method better than simply enriching the bustle gas with natural gas. With these cooler temperatures there is a double carbon benefit not realized at bustle temperatures.
Finally, the hot endothermic gas/natural gas mixture addition at a mixture ratio where furnace burden cooling does not occur will provide a hot upflowing gas to the reducing zone of the furnace. Whereas the addition of natural gas alone provides a cold gas that flows up the center of the reducing zone from the lower cone region, the endothermic gas/natural gas mixture provides a much hotter gas to the furnace center.
In summary, by the invented method, a endothermic gas/natural gas mixture added to or injected into the lower discharge region or cone of a direct reduction furnace provides as much or more carbon content in the product than natural gas alone. The mixture ratio is controlled to prevent burden cooling, and on start-up, the process will speed up the burden heating and initial reduction phase. The invention provides the sought synergistic result; more carbon, no cooling.